Software needed to time limit browsers

Looking for a software to limit the amount of time a user can be on the Internet. Specifically looking for something that when users click on either Internet Explorer or Firefox it will prompt them for a user ID and password. Should also have admin features that you can manage the individual accounts as needed.

Trying to set this up for an environment with adolescents. Tried using Net Time, a Internet Cafe program that the local library uses. The problem with Net Time is that it limits the amount of time people can use the computer in general and not specifically just for the Internet. The kids like to use iTunes and Word so I don’t want to place limitations on those.

Any suggestions or help would be appreciated. Thanks.

David